mechanical fuel pump question

I have a pretty good understanding of mechanical fuel pumps in a carbureted car but one question.When the carburetor fuel bowl is full and the carburetor fuel inlet:runaround::runaround: needle valve is closed, how does this cause the fuel pump diaphragm spring not to release and force more fuel through the outlet valve? Is it just the presence of the fuel in the line to the carburetor, that fuel being unable to go anywhere that prevents the diaphragm spring from releasing?

and some cars have a extra hose coming off the pump or carb going back to the fuel tank for the extra fuel thats not needed

and some cars have a extra hose coming off the pump or carb going back to the fuel tank for the extra fuel thats not needed

This keeps fuel circulating to prevent fuel overheating and vapor lock.

Cars that do not have such a return- line simply have the pump maintain pressure in the line, using a diaphragm and a spring. The pump actually stops pumping when the spring has adequate fuel and pressure in the line.